Need help!! Create percentage inside each class

Hi everybody!
I m writing from Italy and I am not able to go out from a vba problEM
As you can see in the attached file, I have a column of classes , a columns of related revenues and the total for eache class.
I have to create a column, next to these, where I can create the percentage with respect the sum of each class (as you can see by the formula).
My problem is I am not able to say vba to find the totals and to create the percentage just with the revenues related to that total.
Thanks in advance for the help, I really need them.
I cannot understand how to create this.
Thank everybody!!
Max
p.s: I have to work on excel2010